1. Field of the Disclosure
Embodiments disclosed herein relate generally to oligomerization of isoolefins. In another aspect, embodiments disclosed herein relate to oligomerization of isoolefins using an oxygen-containing reaction moderator.
2. Background
Isobutene is commercially significant in many applications. For example, isobutene is one of the comonomers in butyl rubber. Isobutene can also be oligomerized to produce compounds that can be used as chemical feedstock for further reacting or in gasoline blending. Diisobutene, the isobutene dimer, is of particular commercial value in several applications. For example, diisobutene can be used as an alkylation reaction feedstock or as an intermediate in the preparation of detergents. Diisobutene can also be hydrogenated to pure isooctane (2,2,4 tri-methyl pentane) that is highly preferred in gasoline blending.
Isoolefin oligomerization is a catalytic reaction that uses an acid catalyst. For example, oligomerization of isoolefins has been disclosed in U.S. Pat. Nos. 4,242,530, 4,375,576, 5,003,124, and 7,145,049, among others.
When an isoolefin, in particular isobutene, is oligomerized, it is desired to limit the progress of the oligomerization reaction to the dimer stage. High dimer selectivity may be achieved by adding suitable moderator to the reaction mixture, for example, a certain polar component. Typically, oxygenates, such as water, primary, secondary and tertiary alcohols and ethers, are used as the moderator. Use of MTBE as a reaction moderator, for example, is disclosed in U.S. Pat. No. 4,375,576.
The presence of oxygenates makes certain side reactions possible. Such side reactions between the moderator and the isobutene and/or its oligomers leads to formation of heavy oxygenates. Representative moderator side reactions are as follows:Isobutene Dimer+Water→C8 AlcoholIsobutene Dimer+Tertiary Butyl Alcohol(TBA)→C12 EtherIsobutene Monomer+TBA→C8 EtherIsobutene Dimer+Methanol→C9 EtherIsobutene Dimer+Ethanol→C10 EtherIsobutene Trimer+Water→C12 AlcoholIsobutene Trimer+TBA→C16 EtherIsobutene Trimer+Methanol→C13 EtherIsobutene Trimer+Ethanol→C14 Ether
The oxygenated byproducts contained in the selectively dimerized isobutene must be substantially removed prior to further use of the dimers, for example, in alkylation and gasoline blending.
The selectively dimerized isobutene, for example, can be used in an alkylation reaction to produce various derivatives. Alkylation is typically a catalytic process, the efficiency of which depends on the catalyst life and performance. The presence of certain feed impurities, such as oxygenates, can poison the alkylation catalyst, thus adversely affecting its performance and increasing the frequency of expensive catalyst change-outs. Accordingly, oxygenates are not desirable in an alkylation feedstock.
As another example, the selectively dimerized isobutene may also be used in gasoline blending. As above, dimerization products are high value gasoline additives due to their high octane rating. However, the presence of oxygenates in the gasoline fuel can create various environmental, safety, and combustion performance problems due to the nature and properties of certain oxygenates, for example, certain alcohols and ethers. Even though certain ethers can actually improve gasoline combustion, the negative publicity around methyl tertiary butyl ether (MTBE) and other ethers has created a problem for gasoline producers, prompting removal of oxygenates from fuel.
Hydrogenation is typically used to remove oxygenates from the selectively dimerized isobutene. However, hydrogenation requires considerable hydrogen consumption that can add significant operating costs. Also, hydrogenation can often lead to undesirable saturation of olefins, thus resulting in loss of the octane rating and the reduced market value of the selectively dimerized isobutene.
Accordingly, there exists a need for improved methods for producing oligomers of isoolefins.